The present invention relates to liquid stabilizer systems for vinyl halide resin compositions, and to resinous compositions of such products. More particularly, it relates to liquid stabilizer systems including a tin-organo sulfur compound and an additive therefor.
It is well known that vinyl halide resins undergo undesirable changes when they are exposed to heat or to light, and that these changes lead to discoloration and to deterioration of the mechanical properties of compositions containing such resins. Since elevated temperatures are required for the processing and/or formulation of compositions containing these resins, and since the resins are exposed to heat, to light, or both, when they are subsequently used, it is necessary to incorporate in the vinyl halide compositions stabilizers that will inhibit or prevent discoloration, or loss of physical integrity, when they are exposed to such conditions.
Organo tin compounds that contain sulfur have long been recognized as highly effective heat stabilizers for vinyl halide resin compositions. Examples include organo tin ethanol mercaptides such as those disclosed in U.S. Pat. No. 4,059,562; organotin aliphatic mercaptides as disclosed in U.S. Pat. No. 2,726,227; dialkyltin sulfides as disclosed in U.S. Pat. No. 2,746,946; monoalkyltin sulfides as disclosed in U.S. Pat. No. 3,021,302; reaction products of organotin trihalides with sodium alkylmercaptide and sodium sulfide, as disclosed in U.S. Pat. No. 3,442,852; and monomeric tetra-functional organo tin mono- or di-alkyl di- or tri- (alkyl thioglycolate) or (mercapto alkyl carboxylate) compounds such as disclosed in U.S. Pat. No. 5,032,634.
A thorough historical review of organotin and organotin-sulfur compounds in stabilizing polyvinyl chloride compositions appears in the "Plastics Additives and Modifiers Handbook" (J. Edenbaum, Van Nostrand Reinhold, 1992, at pp. 309-326, Chapter 19: Dworkin, "Polyvinyl chloride processing stabilizers: tin and its derivatives".
Combinations of alkyl tin sulfides and alkyl mercaptides are useful liquid stabilizers for vinyl halide resins, but have long been known to exhibit a tendency to form precipitates on formation, or on standing. The precipitates generally have a higher tin content than the liquid product, but are considerably less effective as stabilizers. Thus, the formation of the precipitates represents an undesirable economic loss, in the stabilization efficiency which is lost, in the cost of the product which is lost to the precipitate, as well as in the input of material and energy necessary to attempt to redissolve or otherwise handle the precipitates.
In particular, as pointed out in U.S. Pat. No. 4,059,562, organotin ethanol mercaptides are subject to cloudiness and decomposition in storage. According to that patent, avoiding the cloudiness and decomposition requires formulating the mercaptides with 10 to 40% of a liquid alcohol component that comprises a glycol and an alkyl acid phosphate.
There thus remains a need to formulate liquid stabilizers for vinyl halide compositions, which do not exhibit a tendency to form precipitates and yet which otherwise can be formulated effectively into vinyl halide resin compositions.